Reinforced permeable paving structure

ABSTRACT

A paving structure ( 11 ) of the type having an upper wear layer ( 12 ) and an underlying sub-base layer ( 15 ) of rigid insoluble hard particulate material forming voids for the collection of water permeating through the permeable surface where layer ( 12 ) has an intermediate reinforcing grid ( 16 ) located at an intermediate level of the sub-base layer ( 15 ) such that it is covered by an upper part of the sub-base layer ( 15 U) which is of a thickness not less than 1½ times the dimension of the largest particles in that part of the sub-base layer. A reinforcing grid ( 26 ) at the base of the sub-base layer between this and the sub grade ( 18 ) may also be provided.

[0001] The present invention relates to a reinforced permeable pavingstructure, and in particular to a paving structure which allows theretention or detention of rainfall or other precipitation falling on itor infiltration of collected water into the sub-grade as desired, whilstnevertheless being able to withstand high loads from heavy vehiculartraffic.

[0002] Urban and industrial development results in the almost totalcoverage of the natural ground surface with impermeable materials. Thesemay take the form of buildings (the impermeable surface effectivelybeing the roof of the building), or walkway or roadway surfaces, whichare required for easy transport by wheeled vehicles. A hard smoothsurface capable of withstanding the load applied by vehicle wheelswithout the formation of depressions or ruts is needed for all areaslikely to be the subject of vehicular traffic.

[0003] It is known that in order to prevent pooling during periods ofrainfall such paved or tarmac surfaces must be laid with a ‘fall’ toallow surface water to run off in a predetermined direction to watercollection and/or drainage systems leading to watercourses for thedisposal of storm water during inclement weather. Drainage systems aregenerally built to cope with a maximum expected precipitation, which maybe exceeded from time to time. It is known that meteorological eventssuch as rainfall, although having an ‘average’ value over a period oftime, necessarily involve peaks, which can be classified by thefrequency with which they occur, higher peaks being less frequent.Drainage systems are consequently designed to cope with the peakrainfall which may occur, for example, once every thirty years or onceevery fifty years.

[0004] With the climatic changes which have been occuring in recentyears the assumptions on the basis of which drainage systems have beenbuilt in the past are proving to be incorrect and the failure oroverload of such systems is becoming increasingly frequent. Upgrading ofdrainage systems to cope with increased amounts of run-off is extremelycostly.

[0005] Motor vehicles also introduce another damaging influence due tothe pollution and contamination introduced into the atmosphere duringtheir operation. Pollutants typically caused by motor traffic includeheavy metals, hydrocarbons, rubber dust, silt and other fine detritus,which become deposited on the surfaces of roadways and car parks. Duringfine weather these materials collect and lie on the surface, only to bewashed off in relatively high concentration during periods of heavyrain. Many of these polluting materials are washed into watercourses andfrom there to the sea, polluting both on a long term basis. Even inareas where such run-off is passed through a treatment plant beforebeing released to the natural watercourses a certain proportion of thepollutants nevertheless pass through untreated and, of course, the costof operating such plant has to be borne by the local community.

[0006] Various proposals have been made in the past for amelioratingboth of these problems by the provision of permeable roadways andparking areas which behave in a more natural manner allowing rainfalland other precipitation to pass through the surface into subterraneancollection regions rather than being allowed to run off the surface intodrains. One such proposal is described, for example, in InternationalPatent Application Publication No. WO 96/12067 which describes a pavingsystem having a permeable pavement covering a sub-base layer of mainlyhard nodules, the whole being laid over an impervious membrane toprovide temporary storage in the interstices for chemical spillage orflood water. Pollutants can be treated chemically or decomposedbiologically, and the rate of flow from the storage area can beregulated by providing suitable valve control means.

[0007] The wear surface of the paving system may be permeable tarmachaving passages through it or individual blocks, typically of concreteor other such material, which have passages either within them orbetween them to allow water to pass through rather than being retainedon the surface. The sub-base layer is made from non-friable particulatematerial which, when compacted, retains enough voids between theparticles to hold water up to a given percentage. The sub-base and theunderlying impermeable membrane forms in effect a subterranean cisterncapable of holding a large quantity of water but which is itselfload-bearing and capable of supporting wheeled vehicular traffic. If thesub-grade is suitable the sub-base may be laid directly on it without animpermeable barrier so that water collecting in the sub-base caninfiltrate gradually into the sub-grade.

[0008] One of the problems associated with the known structures lies inthe fact that heavy goods vehicles such as road transport lorries andthe like apply through each individual wheel a load on the surface ofthe ground over which they pass or on which they stand which is muchgreater than the majority of pavement structures are capable ofsupporting. This results in localised displacement of the wear surface,rutting and collapse of the bearing layer. In order to be able tofunction properly and effectively it is necessary for the sub-base layerto be compacted to a point at which the individual stones or particlesinterlock with one another to hold the surface of the layer in asubstantially rigid non-plastic manner, but apart from a compactingoperation nothing can be done to increase the resistance of particles todisplacement under an excessively heavy load. It is essential that thenature of the particles be such that they leave voids between them forthe accommodation of rainfall or other run-off in order for thepermeable pavement to function. Compaction to the point where all voidsbetween the particles are removed, whilst it would increase theload-bearing capacity, conflicts with the requirement for the voids tobe present in order to accommodate the water. This limitation on thestructural strength of the sub-base is layer of this earlier arrangementmakes pavement structures formed according to this earlier arrangementunsuitable for vehicles over a given axle loading.

[0009] The present invention seeks to provide an improved permeablepaving structure capable of withstanding higher loads withoutdetrimentally affecting the storage capacity of the sub-base reservoir,and without requiring the use of more material in the sub-base layer.

[0010] According to one aspect of the present invention, therefore, apaving structure having a system for collecting and retaining or atleast detaining rainfall or other precipitation in an area subject tovehicular traffic, and comprising a permeable surface wear layer and anunderlying sub-base layer of rigid insoluble hard particulate materialis characterised in that a reinforcing grid is located at anintermediate level spaced from the top of the said underlying layer at adepth not less than one and a half times the dimension of the largestparticles in the said underlying sub base layer.

[0011] It is of course known to utilise so-called geogrids to stabiliseloose-laid bulk material.

[0012] It is known for such geogrids to be laid at an interface between,for example, the sub-grade and the sub-base in a roadway structure. TheUS Department of Transportation, Federal Aviation Administration hasproduced a report based on a study of grid-reinforced aggregate basecourses for general aviation airports in which a number of differentcombinations of base courses and geogrid test sections wereinvestigated. Geogrids are deformed or non-deformed grid-like polymericmaterials formed by intersecting ribs joined at the junctions. Geogridsare known for use in reinforcement of foundations, soil, rock, earth orother geotechnical engineering material as an integral part of aman-made project, structure or system. In particular, areas such asgeogrid ballast reinforcement for railroad track bed, reinforcement foraggregate surfaced pavements, and reinforcement for flexible pavementswere investigated. The term ‘flexible pavements’ refers to a structurehaving an asphalt course laid over compacted aggregate layers on a subgrade of relatively low strength as measured by the California BearingRatio (CBR) of 1.5 to 5%.

[0013] On the other hand, paving structures formed in accordance withthe present invention may typically require sub-grade strengths having aCBR of 15% or more. The test results from the above investigation appearto demonstrate that in such circumstances the best improvement isachieved by the use of a geogrid at the interface between the sub-gradeand the sub-base, namely at the bottom of the sub-base. This is thelocation for geogrids in other known applications where, as mentionedabove, they are typically located at an interface between two layers ofa different nature.

[0014] By contrast, in the paving structure of the present invention thegeogrid is not located at an interface between a sub grade and a laidbut is located within the thickness of a constructed sub-base layer.This has been found to bind the larger particles of the sub-base layersufficiently firmly to allow an increase in the weight of traffic usingthe permeable pavement without any damage to the surface by displacementof the particles of the sub-base layer.

[0015] It is preferred that the material of the sub-base layer containsangular elements with well defined edges, in the form of non-roundedparticles of crushed gravel, rock or concrete in a size range up to 100mm. It is preferred that not more than 70% of the sub-base material liesin the range 37.5 mm-100 mm, and preferably not less than 40 % of thesaid underlying material lies in this range. The reinforcing grid ispreferably located at a level not less than one half of the thickness ofthe sub-base layer from the upper surface thereof. Typically, this maybe in the region of 150 mm from the top of a layer in the region of 350mm thick. For thicker sub-base layers greater than this value a secondreinforcing grid of interconnected elements may be provided at a lowerlevel than the said reinforcing grid, and the second reinforcing gridmay be lower than the mid level of the layer. The size of the gridopenings is preferably not greater than the size of the largestparticles of the underlying sub-base layer. In one embodiment the sizeof the grid opening is not greater than the size of the majority of theparticles in the said underlying layer.

[0016] For the best performance using the least material it is presentlyconsidered that the sub-base layer below the said reinforcing gridshould be composed of particulate material in a generally larger sizerange than that in the layer above the said reinforcing grid. It ispreferred that the largest particles of the material of the sub-baselayer below the reinforcing grid are in the region of three times largerthan the largest particles in the sub-base layer above the reinforcinggrid. Likewise the smallest particles in the sub-base layer below thereinforcing grid are preferably not less than twice the size of thesmallest particles in the sub-base layer above the reinforcing grid.

[0017] In a preferred paving structure there is an intermediate layer ofparticulate material between the surface or wear layer and the saidunderlying or sub-base layer. The average particle size of theparticulate material in the said intermediate layer is preferably lessthan the average particle size of the elements of the said underlying orsub-base layer. This intermediate layer may be considered as a so-called‘bedding layer’ which, during construction of the paving structure islaid to a flat, preferably horizontal surface prior to laying theindividual paving blocks or elements which form the wear surface. Theblocks are then vibrated with a vibrator to obtain a flat final regularsurface. The average particle size of the intermediate layer may be inthe region of 2 mm-10 mm, preferably in the region of 5 mm. In oneembodiment the particulate material of the underlying or sub-base layermay have a minimum 10% fines value of 150 K/n. This can be tested inaccordance with British Standard 812 Part 3 and is a measure of theresistance of the material to crushing. The substantial rigidity of thematerial can be tested by establishing that it is non-plastic inaccordance with British Standards Test BS 1377 Test 4.

[0018] The reinforcing grid may be one having a substantiallyrectangular grid structure extending in two orthogonal directions withsubstantially the same resistance to stress in each of the said twoorthogonal directions. The links or arms of the grid may be joinedtogether at intersections to form a substantially laminar sheet, or thegrid may be monolithic. Preliminary stretching of the grid in one orboth directions on manufacture may be undertaken in order to increaseits mechanical strength.

[0019] According to another aspect the invention provides a method oflaying a permeable pavement structure as defined hereinabove, comprisingthe steps of preparing a sub-grade, laying a permeable geotextile orimpermeable membrane thereon, applying a first layer or “lift” of thesaid underlying layer, compacting this layer to refusal with a vibrator,laying a reinforcing grid onto the first layer or “lift” of theunderlying layer, applying a second layer or “lift” of the saidunderlying layer, compacting the underlying layer to refusal with avibrator, laying a permeable geotextile over the said underlying layer,applying an intermediate layer over the said permeable geotextile,levelling the said intermediate layer without compaction thereof,applying a wear layer of individual elements over the intermediate layerand vibrating them and the said intermediate layer into their finalposition with a vibrator.

[0020] The compaction of the sub-base layers may be continued to theso-called point of refusal, that is until further treatment produces thefurther results. This, of course, relies somewhat on the subjectiveassessment of the operation. A degree of certainty can be introducedwith the use of a nuclear Density Meter (a commercially availableinstrument) by the use of which the proportion of maximum compaction canbe measured rather than assessed. It is preferred that the compaction becontinued until reaching 95% of the compacted bulk density achievableunder laboratory conditions.

[0021] Preferably a regulating layer of crushed particulate material theparticle size of which is less than that of the larger particles of thesaid underlying layer but not less than 15% of the size of the largestparticles of the said underlying layer, is applied to the upper surfaceof the second layer or “lift” of the said underlying or sub-base layerprior to compaction thereof whereby to provide a more uniform uppersurface to receive the said permeable geotextile layer.

[0022] Likewise it is preferred that a dressing of clean single sizeangular stone of a size not greater than about 3 mm is applied over theblocks of the wear surface layer prior to the vibration thereof with thesaid vibrator.

[0023] Embodiments of the present invention will now be moreparticularly described by way of example, with reference to theaccompanying drawings, in which;

[0024]FIG. 1 is a cross section through part of an infiltration pavingstructure for disposal of collected water by infiltration to anappropriate sub-grade, and formed in accordance with the principles ofthe present invention;

[0025]FIG. 2 is a cross section through part of an alternativeembodiment of the invention adapted as a source of re-circulated waterfor storage or reuse or for controlled discharge into sewers or streams;

[0026]FIG. 3 is a perspective view of a grid suitable for use in thepavement structure of the present invention; and

[0027]FIG. 4 is a cross section through a part of a further alternative,and presently preferred, embodiment of the invention.

[0028] Referring first to FIG. 1 a paving structure generally indicated11 comprises an upper layer of blocks 12 which may be of the typedescribed in the Applicant's co-pending International Patent Applicationpublished under no. WO 99/64680 the disclosure of which is incorporatedherein by reference, which are substantially is impermeable, but havegrooves or channels in one or more lateral edges thereof to providedrainage passageways from the top to the bottom. In addition to an upperbevel which can be seen in the drawings, part of the upper side wall istapered along the entirety of the edge between the upper surface and thelateral surface to allow a small degree of flexing of the overallsurface by movement of the blocks upon the passage of heavy traffic.This helps to avoid spalling, and the channels provided by adjacenttapered surfaces also encourage the drainage of rainwater from thesurface through the drainage channels into the underlying layers to bedescribed in more detail below.

[0029] The blocks 12 are laid on an intermediate layer or bedding course13 of fine particulate or granular material of a size in the region of 2mm-10 mm, preferably up to 5 mm, which in turn is laid to tolerance on ageotextile membrane 14 itself overlying a sub-base layer generallyindicated 15. The bedding layer is raked and levelled before the blocks12 are laid on it, and blocks 12 are laid directly on the bedding layer13 with no grouting or other filling (such as sand) either betweenthemselves and the layer 13 or between each other so that there are nofine materials to wash down into the lower layers of the structure whenrainfall infiltrates the passages between the blocks. After laying theblocks a vibrator is passed over the entire surface to settle the blocksand ensure they all lie to a common surface. Before or after this isdone the block-paved surface may be dressed with a thin layer of fineclean stone in a size range about 2 mm-3 mm. These stones are thenbrushed into the interstices and help to lock the blocks in positionagainst relative movement without clogging the passages through whichthe water drains into the underlying layer.

[0030] The sub-base layer 15 is composed of crushed gravel, rock,concrete or other hard insoluble particulate material havingwell-defined edges. It must be sound, clean, and non-friable and freefrom clay or other fine particulate material. This property allows thecompaction of a layer typically in region of 350 mm to 400 mm thick, toa state in which it is capable of bearing the load of vehicular trafficsuch as motor cars, trucks and lorries. For this purpose the materialmust be non-plastic when tested in accordance with BS1377 Test 4. Thematerial must also have a minimum 10% fines value of 150 K/n when testedin accordance with BS812 Part 3. In conducting such tests the samplesmust not be oven dried and should be soaked in water at room temperaturefor 48 hours before the test is conducted. This ensures that there areno variations between the performance of the material when wet and whendry as it must pass the test when effectively saturated.

[0031] The dimensions of the particles in the sub-base layer 15 may beup to 100 mm with up to 60% of the material being less than 37.5 mm andnot more than 40% of the material being greater than 37.5 mm. Up to 20%of the material may be less than 20 mm with only 5% being less than 10mm. This ensures that the material is permeable and, when compacted,nevertheless has a large proportion of void space between the particles.Typically 30% of the volume occupied by the layer 15 will be void spacewhich is available for receiving water when the permeable pavingstructure is in use.

[0032] In order to enable the sub-base structure to carry heavier loadsa reinforcing grid 16 is located at an intermediate level, at a distancetypically 150 mm from the top of the sub-base, and in any event not lessthan one and half times the maximum particle size from the top of thesub-base layer to ensure that an adequate cover over the grid 16 isprovided. In this case, the depth of the grid from the surface,indicated D in FIG. 1, is in the region of 150 mm, although it may be alittle deeper with a positioning error of +/−10-15%. The overallthickness of the sub-base layer 15 may typically be in the region of 350mm although greater or lesser thickness may be used if circumstancespermit or dictate. This may be laid in two operations or ‘lifts’ with alower layer being spread and preliminarily compacted first before thegeogrid 16 is laid over it; the upper lift then spread over that. Finalcompaction to the required density state then follows.

[0033] Beneath the sub-base 15 is a geotextile layer 17 which separatesthe sub-base 15 from the sub-grade 18 which preferably should have a CBR(California Bearing Ratio) of at least 15%.

[0034] The geogrid 16 is preferably a polymeric plastics material ofhigh strength, with a grid size typically in the region of 40 mm andreinforced junctions between the intersecting ribs. Grid sizes up tovalues in the region of 100 mm may also be utilised. Dimensions lessthan 40 mm are unlikely to be effective in allowing the necessaryinterlock between the upper and lower layers or ‘lifts’ of the sub-base15.

[0035] It is believed that the reinforcement effect is achieved byforming in effect an intermediate stratum within the sub-base 15 whichis more resistant to the relative movement of the particles of which itis composed than the remainder thereof due to the fact that particles inthe upper layer or ‘lift’ can project down through the openings in thegrid and also between the faces of other particles in the lower layer or‘lift’, some of which project upwardly through the grid such that theforces exerted by wheeled traffic, and which might otherwise cause alateral displacement of material beneath and to one side of it as itpasses, are less able to cause such displacements due to the additionaltensile effect of the ribs holding the particles of this intermediatestratum in place. Because the grid is located in the region of one andhalf times the maximum particle size from the top of the sub-base layerthis interlock effect is achieved over substantially the entirety of thethickness of the upper lift of the sub-base, penetrating some distancebelow the grid, so that the intermediate stratum within the sub-baseeffectively acts as a stiffening layer of rigid material even though theparticles have up to 30% void space between them. The upper layer or“lift of the sub-base 15 is compacted to refusal, that is compaction iscontinued until the point where further treatment has no further effect,or, if a Nuclear Density Meter is used to measure compaction, to a pointgreater than 95% of the laboratory achievable maximum. Even so the uppersurface of the sub-base layer is very irregular with asperities andcavities due to the presence of the relatively large stones (up to 100mm) in the material. In order to regularise this surface aregularisation layer 15 a of clean crushed stone the particles of whichare not greater than 20 mm and not less than 5 mm is applied to the topof the sub-base during or just before the vibrator is passed over it.

[0036] Rainfall or other precipitation (when melted) falling on theupper surface of the blocks 12 can infiltrate through the wear layer andthe intermediate or bedding layer 13 which acts to trap many of thepollutants carried by the water. The effective storage volume of thesub-base layer allows the water to collect in this region, and then bediffused more gradually through the sub-grade which, in this embodiment,is assumed to be porous or to have sufficient faults to allow the waterto permeate either through the ground downwardly or laterally throughthe edges of the storage region thus formed. The nature of the sub-basematerial 15 is such that, even when drained, the particles retain somemoisture in pockets which ensures a humid atmosphere suitable for thegrowth of bacteria which can migrate upwardly through the geotextile 14into the region of the bedding layer 13 to attack and break down certainof the pollutants trapped therein. Thus, as well as serving as a stormwater control system the paving structure of the present invention alsoacts via bioremediation to degrade the entrapped hydrocarbons and otherpollutants, which, together with the filtering action of the geotextile,cleans the water passing through the system. It is estimated that onesquare metre of the paving structure described above will cause up to 70grams of oil per annum to be degraded and the water discharging from thestructure, while not potable, can be used for many secondary purposessuch as flushing lavatories, washing floors and cars or wateringvegetation.

[0037] The collection of water for recycling can be enhanced byutilising the embodiment of FIG. 2 which largely corresponds to that ofFIG. 1 except that the geotextile 17 at the interface between thesub-base 15 and the sub-grade 18 is replaced by an impermeable membrane19 which, as well as underlying the sub-base 19 also passes up the sidesto the surface, terminating flush with the upper surface of the wearlayer 12. An outlet pipe 20 from a manifold collector 21 then allows thewater contained in the reservoir constituted by the sub-base 15 to bedelivered for such secondary purpose as is appropriate.

[0038]FIG. 3 illustrates a typical geogrid suitable for use in thepaving structure of the invention. It comprises a monolithic grid-likestructure of longitudinal ribs 22 separated by regularly spaced lateralor transverse ribs 23. At the nodes or junctions between ribs there isan enlarged boss 24 and the ribs are stretched after moulding toorientate the molecules and increase the tensile strength thereof

[0039] Another, presently preferred, embodiment is illustrated in FIG.4. In this Figure the same reference numerals have been used to identifythe same or corresponding components as in the embodiments of FIGS. 1and 2. In this embodiment, however, the parts of the sub-base formedabove and below the intermediate geogrid 16 although of the samematerial are in different size ranges, the lower sub-base layer 15Lbeing formed of stone in the range 63 mm to 10 mm and the upper layer15U being in the range 20 mm to 5 mm. In each case the stone is evenlygraded, that is to say there is a roughly equal proportion of stone ofall sizes within the size range and no preponderance of, say, the largeror the smaller end of the range. In this embodiment the largest stonesare somewhat smaller, even in the lower layer, than those used in theembodiments of FIGS. 1 and 2, and the smallest fraction is above 10 mmwhereas in the embodiment of FIG. 1 up to 5% of the material could beless than 10 mm.

[0040] The geotextile 17 at the interface of the sub-base with thesub-grade of the FIG. 1 embodiment is replaced with a geogrid 27 whichmay have the same properties as the geogrid 16 illustrated in FIG. 3.

1. A paving structure having a system for collecting and retaining ordetaining rainfall or other precipitation, in an area subject tovehicular traffic, comprising a permeable surface wear layer (12) and anunderlying sub-base (15) layer of rigid insoluble hard particulatematerial, characterised in that a reinforcing grid (16) is located at anintermediate level spaced from the top of the said underlying layer at adepth not less than one and a half times the dimension of the largestparticles in that part of the said underlying layer (15) above the saidreinforcing grid.
 2. A paving structure according to claim 1,characterised in that the reinforcing grid (16) is located at a levelnot less than one half of the thickness of the said underlying sub-baselayer (15) from the top thereof.
 3. A paving structure according toclaim 1 or claim 2, characterised in that there is provided a secondreinforcing grid (25, 26) of interconnected elements at a lower levelthan the said reinforcing grid (16).
 4. A paving structure according toany preceding claim, characterised in that the size of the grid openingsis not greater than the size of the largest of the particles of the saidunderlying sub-base layer (15).
 5. A paving structure according to anypreceding claim, characterised in that the grid size is in the region ofthe mean value of the size of the particles in the said underlyingsub-base layer (15).
 6. A paving structure according to any of claims 1to 7, characterised in that the grid size is in the region of 40% of themaximum particle size of the said underlying sub-base layer (15).
 7. Apaving structure according to any of claims 1 to 6, characterised inthat claim 1, in which the material of the underlying sub-base layer(15) comprises non-rounded angular particles with well defined edges ofcrushed gravel, rock or concrete in a size range of up to 100 mm withnot more than 5% thereof being less than 10 mm.
 8. A paving structureaccording to any of claims 1 to 7, characterised in that not less than40% of the material of the said underlying sub-base layer (15) lies inthe range 37.5 mm to 100 mm.
 9. A paving structure according to anypreceding claim, characterised in that not more than 70% of the materialof the said underlying sub-base layer (15) lies in the range 37.5 mm to100 mm.
 10. A paving structure according to any of claims 1 and 9,characterised in that the sub-base layer below the said reinforcing gridis composed of particulate material in a generally larger size rangethan that in the layer above the said reinforcing grid.
 11. A pavingstructure according to claim 10, characterised in that the largestparticles of the particulate material in the sub-base layer (15L) belowthe said reinforcing grid (16) in the region of are three times largerthan the largest of articles in the sub-base layer (15U) above thereinforcing grid.
 12. A paving structure according to claim 11,characterised in that the smallest particles in the sub-base layer (15L)below the said reinforcing grid (16) are not less than twice the size ofthe smallest particles in the sub-base layer (15U) above the reinforcinggrid.
 13. A paving structure according to any of claims 10 to 12,characterised in that the material of the lower sub-base layer (15L)comprises non-rounded angular particles with well defined edges ofcrushed gravel, rock or concrete in a size range from about 63 mm toabout 10 mm.
 14. A paving structure according to claim 13, characterisedin that the material of the upper sub-base layer (15U) comprisesnon-rounded angular particles with well defined edges of crushed gravel,rock or concrete in a size range from about 20 mm to about 5 mm.
 15. Apaving structure according to any preceding claim, characterised in thatthere is an intermediate layer (13) of particulate material between thesurface or wear layer (12) and the said underlying sub-base layer (15).16. A paving structure according to claim 15, characterised in that theaverage particle size of the material in the said intermediate layer(13) is less than the average particle size of the elements of the saidunderlying sub-base layer (15).
 17. A paving structure according toclaim 16, characterised in that the average particle size of theintermediate layer (13) is in the region of 2 mm-10 mm, preferably inthe region of 5 mm.
 18. A paving structure according to any precedingclaim, characterised in that the particulate material of the saidunderlying sub-base layer has a minimum 10% fines value of 150 K/n. 19.A paving structure according to any preceding claim, characterised inthat the material of the said underlying sub-base layer (15) issubstantially non-plastic.
 20. A paving structure according to anypreceding claim, characterised in that the said reinforcing grid (16) isone having a substantially rectangular grid structure extending in twoorthogonal directions with substantially the same resistance to stressin each of the said two orthogonal directions.
 21. A reinforced pavingstructure according to any preceding claim, characterised in that thegrid (16) is a polymeric plastics material composed of a plurality oflinks or ribs joined together at intersections to form a substantiallylaminar sheet.
 22. A paving structure according to any preceding claim,characterised in that an upper stratum of the said underlying layer hasan additional component (15 a) of particles the maximum dimension ofwhich is a fraction of the maximum dimension of the largest particles inthe said underlying sub-base layer (15).
 23. A paving structureaccording to claim 22, characterised in that the said fraction is notgreater than 60%, preferably not greater than 40% and more preferablynot greater than 20%, and in any event not less than 15% of the size ofthe largest particles in the said underlying sub-base layer (15).
 24. Apaving structure according to any of claims 15 to 18, characterised inthat the surface wear (12) layer is composed of individual elements orblocks laid on the intermediate layer (13) with no grouting, sand orother filling between them or between them and the intermediate layer(13), and a dressing of clean stone not greater than 3 mm is overlaidand worked into any interstices between them.
 25. A method of laying apaving structure according to any preceding claim, characterised in thatcomprises the steps of: preparing a sub-grade; laying a permeablegeotextile (17), impermeable membrane (19), or geogrid (26) thereon;applying a first layer or “lift” (25L) of the said underlying sub-baselayer (15); compacting this layer to refusal with a vibrator; laying areinforcing grid (16) onto the first layer or “lift” (15L)of theunderlying layer (15); applying a second layer or “lift” layer (15U) ofthe said underlying sub-base layer (15); compacting the underlyingsub-base layer to refusal with a vibrator; laying a permeable geotextile(14) over the said underlying sub-base layer (15); applying anintermediate layer (13) over the said permeable geotextile (14);levelling the said intermediate layer (13) without compaction thereof;applying a wear layer (12) of individual elements (10) over theintermediate layer, and vibrating them and the said intermediate layer(13) into their final position with a vibrator.
 26. A method as claimedin claim 20, in which a regulating layer (15A) of crushed particulatematerial the particule size of which is less than that of the largerparticles of the said underlying sub-base layer (15) but not less than15% of the size of the largest particles of the said underlying sub-baselayer (15), is applied to the upper surface of the second layer or“lift” (15U) of the said underlying layer (15) prior to compactionthereof whereby to provide a substantially uniform upper surface toreceive the said permeable geotextile layer (14).
 27. A method asclaimed in claim 21 or claim 22 in which a dressing of clean stone in asize range not greater than about 3 mm is applied over the elements (10)of the wear surface layer (12) prior to the vibration thereof with thesaid vibrator.